Recently, internal combustion engines for motor vehicles have frequently been equipped with electronically controlled fuel injection arrangements to solve problems regarding harmful components in the exhaust gas, efficiency of fuel consumption and the like. Such an electronically controlled fuel injection arrangement is designed in a manner so that the changes in the operating state of the engine with respect to a load, a rotating speed of the engine, a temperature of cooling water or coolant, the amount of intake air and the like are detected as electrical signals which are used as inputs to the fuel injection apparatus, and then this apparatus outputs a fuel injection signal to control the injector. During a transient state at the start of a vehicle or the like, fuel injection is controlled independently of the fundamental injection, namely asynchronously with respect to the fundamental fuel injection, in order to improve a response speed in the transient state.
Examples of the asynchronous fuel injection control methods have been disclosed in the Official Gazettes of Japanese Patent Unexamined Publication Nos. 68530/1984, 14232/1978, and 136523/1975. The asynchronous fuel injection control method as shown in the flowchart of FIG. 1 has also been known.
The above-mentioned conventional fuel injection control method will now be described with reference to the flowchart shown in FIG. 1. In step 200, the engine is started. In step 202, a check is made to see if the idle contact of the throttle sensor has switched from ON to OFF. If not, fuel injection control in the ordinary manner is performed (in step 208). On the other hand, if it is determined in step 202 that the idle contact has switched off, a comparison is made at 204 to see if the rotating speed (N.sub.e) of the engine is higher than a predetermined rotating speed (N.sub.u). If the speed (N.sub.e) is found to be equal to or less than the speed (N.sub.u) in step 204, fuel injection control in the ordinary manner is performed (in step 208). On the other hand, if it is determined in step 204 that the actual speed (N.sub.e) is above the predetermined speed (N.sub.u), asynchronous fuel injection control to increase the amount of fuel injected is executed in step 206 in order to produce a rich air-fuel mixture and improve the response speed in the transient state.
However, in such a conventional electronically controlled fuel injection apparatus, the execution of the asynchronous correction to increase the amount of fuel injected meets only the particular situation corresponding to a part of the operating state of the engine. Therefore, in small-sized and light-weight vehicles, vehicles having a small cylinder capacity, or the like, the explosion pressure is increased to a higher level than needed, due to the rich air-fuel mixture, which causes problems such as large shocks which momentarily occur in the vehicle and cause the driver to have an unpleasant feeling. Also, when the accelerator is frequently pressed little by little or the like, there occur problems such as the engine condition becoming bad and the engine stopping, or the spark plug becoming dirty and being damaged. Further, front wheel drive vehicles are likely to be influenced due to a small shock as well. Therefore, there is need for a fuel injection control method which does not exert an adverse influence on the running state of vehicles such as small-sized and light-weight vehicles, vehicles having a small cylinder capacity, front wheel drive vehicles, or the like.